Trophic dynamics of two omnivorous shrimps in the middle Paraná River

Abstract

Research on how hydrological regimes affect the attributes of floodplain systems is fundamental to understanding ecosystem functioning. The shrimps Macrobrachium borellii (Nobili, 1896) and Palaemon argentinus (Nobili, 1901) are sympatric omnivorous species that are important components in the trophic webs of the Paraná River. This river presents a vast floodplain dominated by C3 plants, which in periods of high water (HW) are available as a trophic resource. We aimed to estimate the contribution of trophic resources to shrimp diets using stable isotope analysis of muscle and hepatopancreas and stomach content analysis. We expected that shrimp diets would vary between high and low water (LW) and that C3 plants would be an important food source for both shrimp during high water, especially for the larger and more agonistic shrimp species M. borellii. We also hypothesized that the two species consume prey at different trophic levels to avoid competition; due to the behavioral differences, we expect the niche of P. argentinus to be smaller than that of M. borellii. We found that both species consume C3 plants and that, unlike P. argentinus, M. borellii rapidly exploits the available C3 plants during HW. The niche breadth of M. borellii is generally larger than that of P. argentinus, while the niche overlap between both species is smaller during LW. The results obtained highlight the role of shrimp in aquatic food webs, as they can exploit C3 carbon sources during HW and make them available to other aquatic consumers.